Sydney's Age

In the beginning of the film, Sydney (Katharine Hepburn) states that she never met her father. She was born after he was away at a mental institution. Later they say he was gone for 15 years. This would put Sydney at 15 years old. She's talking about marrying someone. This doesn't add up. I tried to research the original story & wasn't able to verify the time period. Wikipedia says he was gone for 20 years, which would make more sense, but in the film they clearly say he was gone for 15.